1. Field of the Invention
This invention relates to couplers for interfacing the telephone network with terminal equipment and more particularly to such couplers incorporating signal power limiting. The invention further concerns a process for limiting the signal power transmitted to the telephone network by terminal equipment interfaced therewith.
2. Description of the Prior Art
Federal Communications Commission Rules Part 68 entitled "Connection of Terminal Equipment to the Telephone Network" provide uniform standards for the protection of the telephone network from harm caused by the connection of terminal equipment thereto. When a customer desires to connect terminal equipment which has been registered, he is required to notify the telephone company of each telephone line to which he intends to connect such equipment. The telephone company, after determining the attenuation of each such telephone line between the interface and the telephone company central office, will make such connections as are necessary in each so-called programmed data jack which it will install at the interface, so as to allow the maximum signal power delivered by such terminal equipment to the telephone company central office to reach, but not exceed, the maximum allowable signal power permitted at the telephone company central office.
The programmed data jack incorporates a programming resistor. The proper programming resistor (Rp) is to be selected by the telephone company at the time of installation based upon the loop loss of the telephone line to program the signal power output of the terminal equipment so as to arrive at an optimum signal power level of -12 dBm at the central office. Table 1 reproduced from the Federal Register Vol. 41, No. 134 (July 12, 1976) gives the required resistance value for the programming resistor for each value of the desired signal power level in dBm.
TABLE 1 ______________________________________ Programmed Data Equipment Programming Resistor (Rp) Signal Power Output ______________________________________ short 0 dBm 150 ohms -1 dBm 336 ohms -2 dBm 569 ohms -3 dBm 866 ohms -4 dBm 1,240 ohms -5 dBm 1,780 ohms -6 dBm 2,520 ohms -7 dBm 3,610 ohms -8 dBm 5,490 ohms -9 dBm 9,200 ohms -10 dBm 19,800 ohms -11 dBm open -12 dBm ______________________________________
Terminal couplers are known which provide means whereby customer-provided automatic terminal equipment may be connected to the switched telecommunications network for data and voice communications. One such coupler is manufactured by the Elgin Electronics Inc. as the EDC 1001 A Automatic Data Coupler and described in Elgin System Practice Bulletin 20011 (July 1972), Issue 3. When the connections are completed, the data coupler must be adjusted internally to limit the customer signal power to a level which will not exceed a -12 dBm signal level at the serving central office.
The prior art connected the programming resistor directly into the telephone line as apart of an attenuator. The disadvantage with this arrangement is that with the resistor in the signal line bidirectional attenuation results, that is, both the received and transmitted signals are attenuated. Moreover, any noise induced into the attenuator leads will be added to the throughgoing signal.
No terminal coupler is known where the programmable resistor controls only the transmitted signal and/or the programming resistor is not part of the signal circuit.